Precast pre-stressed concrete piles are used in the construction of foundations for buildings, bridges, wharfs, docks and other structures. Concrete pile lengths are limited by the lengths that can be shipped to the construction site and lifted by the pile driving machine. In many cases to develop the needed bearing capacity the pile lengths are greater than what can be shipped and handled by the pile driving machine. In order to achieve the required length precast concrete piles are driven as spliced segments. The International Building Code requires the splice to develop a minimum of 50% of the un-spliced pile capacity in moment (bending) and tension.
Concrete has large compressive strength, but little tensile strength. The tensile strength governs the moment or bending capacity of the pile. In order to make the pile rigid enough to handle, ship and lift into the pile driving machine a system of high strength cables or strands is used to develop the needed moment and tensile strength. The number and size of the pre-stressing strands is dependent on the size of the pile.
Currently there are several splices available in the market. Most of these splices consist of a splice plate on the adjoining ends of the pile segments to be spliced. These plates are attached to the ends of the pile segments with reinforcing steel welded to the plates and protruding into the concrete of the pile segments. These plates are mechanically joined by systems of wedges, pins, and other mechanical means.
As concrete has little tensile strength the pre-stressing strands are necessary to develop tensile strength required for moment or bending stresses, and for tension loads. The pre-stressing operation uses abutment at both ends of pile forms. At one end the strand is held with strand chucks and at the other end a calibrated hydraulic jack is used to stress the strand. After the strands are stressed (pulled) the concrete is placed in the pile forms. When the concrete has reached a specified strength the strands are released from the abutments and the pre-stress forces are released into the concrete pile. The bond between the concrete and the strand requires a developmental length of some significant length. Because of this the ends of the concrete pile segments have little or no pre-stress force. Due to this lack of pre-stress force in the ends of the joined segments of the pile, the spliced pile does not develop any moment or tension at the splice. To address this, additional reinforcing steel is added to the ends of the pile segments to the joined.